1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to the use of a plastic material in focusing lens members of the zoom lens.
2. Description of the Prior Art
There have been proposals for using lenses of a plastic material in the focusing members of the zoom lens in U.S. Pat. Nos. 3,920,315 and 3,972,592. These proposals are directed toward employment of such material in the zoom lenses for projectors. The disclosure of the first-named patent applies to use in the optical compensation type, and the disclosure of the second-named patent applies to use in the varifocal lens, so that in a rigorous sense, zooming accompanies shifting of the image plane. Therefore each time focusing is followed by zooming, the focusing members must be re-adjusted in position. Also, atmospheric or externally influencing factors, for example, surrounding temperature change and humidity change, have not been taken into account in all the previous proposals.
On the other hand, it is known to provide a fixed-focal length lens for photography using plastic lenses in the focusing members in Japanese Laid-Open Patent Application No. SHO 55-143518. In this case, the image shift resulting from the temperature change is reduced by combining a plurality of plastic lenses and by setting forth specific conditions for paraxial arrangement.